Because of the nature of the applications to which various corrugated board and paper products are put, the products are frequently subjected to forces in excess of those which the products can be reasonably expected to withstand. Grocery bags are one product illustrative of the problems which can be encountered. One material of which grocery bags can be made is kraft paper. Frequently, such products are manufactured giving them a double thickness in order to limit the risks of bag failure by ripping when a bag becomes overloaded. In some instances, grocery bags are made of special heavy duty kraft paper in attempting to provide then with the necessary strength. When double thicknesses of kraft paper are used, however, an inordinate cost can be incurred.
Even with bags strengthened as described above, adequate strength is not always achieved. Such inadequacies are highlighted by the fact that grocery stores frequently insert one manufactured bag inside another prior to filling a bag with groceries.
The problems are similar in the case of corrugated board boxes. Generally, corrugated cardboard boxes are stronger than paper products and able to withstand stronger forces. In some respects, this additional strength can be attributed to the corrugation interposed between adjacent substrates to the box. Typically, however, the users of the end products tend to overload them to a point at which they are unable to withstand the force applied.
The examples articulated thus far are two typical examples of which the average layman is well aware. Other applications do, however, exist for paper and corrugated board products. Sheets of corrugated board are used to form corrugated cardboard pallets for use in material handling. A typical pull sheet is made dimensioned four feet by four feet, and a score line is impressed into the sheet extending generally parallel to, and proximate, one of the edges. A flap thereby formed between the score and the edge to which the score is proximate is able to be pivoted about the score and can serve as a lip by which the sheet can be grabbed. Lift trucks used to manuever the commodities carried by the pallets typically include clamps which can grab onto the flap of the pull sheets to pull the pallet onto a flat lift plate on the lift truck. Frequently, the pallets carry commodities in bags, drums, or other containers which contain materials weighing in excess of one ton. As can be seen, even absent the reduced strength which is present at the score line, the pull sheets are subjected to significant forces. When a score as described above is present, however, the pulling of the pallet by the clamping mechanism on the lift truck effects a concentration of stresses at the score line.
In recognizing the shortcomings various companies have attempted to particularly strengthen the pull sheets at the score line. It has become common practice to interpose an extra lamina into the sheet overlapping with the score. Such a lamina is typically a strip of strong kraft liner board extending along the scoring. As in the case of paper bags, the expense added to the cost of the product because of the extra cost of the kraft paper can be significant. Even more importantly, however, overall production expenses are significantly raised because of the additional time involved in being required to insert this strip during the production process.
It has been recognized that both paper and corrugated board products can be reinforced with filamental strands and tapes which are coated with an adhesive to bond the reinforcing material network to the paper or board to which it is applied. Products reinforced in this manner generally allow the use of lighter web substrates and afford greater load carrying capacities. Even with such reinforcing networks, however, problems yet exist. Typical networks known in the art comprise strands or tapes which provide strength primarily in one direction. This is so because they are aligned generally in straight parallel lines. In the case of corrugated board pull sheets, the strands or tapes are made to extend substantially perpendicular to the flutes of the corrugation. The product thereby formed in given an increased strength in a direction perpendicular to the flutes (that is, in a direction in which the reinforcing strands or tapes extend). The network formed by the reinforcing material, however, gives little, if any, increased strength in a direction transverse to the direction in which the strands or tapes extend. Although an improved resistance to tearing in a direction parallel to the flutes is imparted to the product, tearing in a direction generally perpendicular to the flutes can still readily occur.
H. B. Fuller Company, the assignee of the present application, has recognized the advantages which can be obtained in placing reinforcement filaments in serpentine patterns across the corrugated board or paper substrates. U.S. patent application Ser. No. 047,255 is for a Web Reinforced with String-Type Adhesive and Method of Manufacturing Same. That application discloses a reinforcing network utilizing serpentine patterns of reinforcing filaments. Such networks provide additional strength in directions both parallel to and perpendicular to the directions in which the flutes of the corrugation extend. Although the structure and method of the above-referenced application is a significant improvement over the prior art relevant to that application, a number of deficiencies still remain. Since the guide bar which carries the guide eyelets for movement across the direction in which the substrates are made to move is operated independent of the mechanism for moving the substrates, extremely close coordination must be maintained between the movements of the substrates and the guide bar in order to insure that the wave of reinforcing filament will have a constant frequency. In order to insure this coordination, the substrates must be passed through the double backer pressure application device at a slower speed than might otherwise be allowed.
Additionally, problems can be encountered in obtaining the desired amplitude of the wave formed. If the reinforcing network forming machine is located in a narrow room, space might not be available to permit the desired transverse movement of the guide bar in order to achieve a wave having the amplitude sought.
It is to these deficiencies in the prior art that the invention of the present application is directed. It provides apparatus for forming a network which strengthens paper and cardboard products in a multiplicity of directions, and the present application includes claims directed to the products thereby created. The claims of the present application are also directed to a process for forming such products.